Conventional material processing techniques are limited to the production of materials close to the equilibrium state with very large grains and the loss of nanostructure dimensionality.
Conventional powder metallurgy processing techniques typically prepare materials from a melt or using powder metallurgy techniques, such as hot pressing or sintering, and produce materials with relatively large grains crystallites) and result in the loss of nanostructural properties. Materials design and processing approaches at or close to the equilibrium state can impose limitations on the properties of resulting materials. The production of multiphase composites is difficult or impossible using such techniques. Fabrication of metal matrix, metal-ceramic or reinforced ceramic composites is considered industrially important however, owing to a wide range of applications such as gas turbine and piston engine parts, orthopedic implants, transparent laser materials, remote sensing, chem-bio detection and space vehicles. The difficulty of controlling metastable phases within the composites, the loss of filler nanostructure dimensionality and aggregation of dopant or damage in the bulk materials limits mass production and use.
Current methods of processing utilizing shockwave consolidation are limited to shapes and sizes that are amenable to gas gun and explosive consolidation systems.
Ceramic materials have been an active field of research over the last few decades owing to excellent physical properties including high temperature stability, chemical inertness, strength, and wear resistance. This makes them suitable for a wide range of applications when many conventional metallic components would fail. The major limitation to use is brittleness, which restricts use in structural applications, and thus a significant effort has been placed in increasing toughness through the use of second phase reinforcements such as particulates and long/short fibers. Similarly, the incorporation of carbon nanotubes (CNTs), analogous to traditional micrometric fibers, is aimed at enhancing fracture toughness.
It is possible to incorporate carbon nanotubes (CNTs) and carbon nanofibers into 3Y-TZP ceramics using HIP or Spark Plasma Sintering (SPS), resulting in enhancement of mechanical and electrical properties. No significant enhancement is observed in fracture toughness, fracture strength or hardness however.
Tetragonal stabilized zirconia with 3 mol. % yttria (3Y-TZP) was down selected due to its relatively high strength and fracture toughness, ideal for many structural applications. It differs from other ceramics due to a stress-induced phase transformation. ZrO2 has three different crystallographic forms depending on temperature. It exhibits a transformation toughening mechanism due to transformation from the monoclinic to the tetragonal phase that increases crack propagation resistance. This mechanism makes ZrO2 much stronger compared to all other ceramic materials. Hence the true benefits of the reinforcement (second phase) n improving the mechanical properties of ceramic nanocomposite processed using thermal techniques is not clearly established.
Ceria is viewed as one of the most promising alternatives to yttrium stabilized zirconia for use in high temperature solid oxide fuel cells (SOFC) allowing lower temperature operation. Extensive development of ceria-based electrolytes has taken place. Super-ionic conductivity in samarium doped ceria nanocomposites is both theoretically and experimentally possible. This behavior may arise not from the SDC or Na2CO3 bulk, but from the interface between two constituent SDC and Na2CO3 phases, as illustrated in FIGS. 2A-B.
Solid sodium carbonate is a fast ionic conductor with negligible electronic conductivity at higher temperatures. The conductivity increases with increasing temperature. The pure salt exhibits a transition into a high temperature phase accompanied by a change of thermal expansion, rendering the production of al ng-term thermally stable sintered layer difficult. Na2CO3 is ferroelastic, developing a spontaneous strain below a phase transition from paraelastic to ferroelastic. Ferroelastic materials are defined as exhibiting multiple domains, or twins, which may be switched on with application of external stress. Such domain microstructures often result from phase transitions.
Na2CO3 has three synthetic polymorphs: two monoclinic (β and γ) and one hexagonal (α); these are stable in different temperature ranges. At 489° C., Na2CO3 undergoes β-monoclinic to α-hexagonal (C2/m P63/mmc) ferroelastic phase transition that is driven by a planar elastic instability. The transition is ideal, continuous m=2 ferroelastic phase transition and it is reported to show complete lattice melting predicted by Landau theory. Since the melting point (851° C.) is much higher than the ferroelastic transition, it is termed as “melt-like phase.” These phase transitions have a significant effect on the stability and conductivity of core-shell nanoparticles. Amorphous carbonate in the SDC-Na2CO3 composites may be tightly bonded to the surface of SDC nanocrystals to form an intimate shell layer via a long-range interface interaction, thereby minimizing crystalline Na2CO3 formation. This may be responsible for enhanced transport properties of oxide ions in the SDC-Na2CO3 composite electrolyte.
Extensive research has been devoted to stabilizing the high temperature phase at lower ambient temperatures by doping and high pressure to enhance conductivity, but without much success. High density forms are required for application in SOFC or other electrochemical cells.
Needs exist for improved material manufacturing methods.